A spark plug for igniting a mixture gas with a spark discharge is used conventionally for an engine or an internal combustion engine for a motor vehicle. Recently, there is a demand for higher output and lower fuel consumption of an internal combustion engine. Accordingly, development is in progress for a plasma-jet spark plug capable of providing faster propagation of combustion, and igniting a lean mixture gas for higher ignition limit air fuel ratio (cf. patent document 1, for example).
Patent document 1: JP 2007-287666 A
The plasma-jet spark plug has a structure including a discharge space (cavity) of a small volume formed by an insulator, such as ceramic insulator, surrounding a spark discharge gap between a center electrode and a ground electrode. In one example of the ignition method of the plasma-jet spark plug, at the time of ignition of a mixture gas, a spark discharge is performed first by applying a high voltage between the center and ground electrodes. Due to the resulting dielectric breakdown, a current with a relatively low voltage can flow in the gap between the center and ground electrodes. Accordingly, a plasma is formed in the cavity by changing the discharge state by the supply of electric power between the center and ground electrodes. By ejecting the thus-formed plasma through a communication hole (so-called orifice), the plasma-jet spark plug performs an ignition to a mixture gas.
However, since the plasma-jet spark plug requires the supply of energy in a large quantity in order to produce plasma, the plasma-jet spark plug is inferior in the durability as compared to the conventional spark plug. Moreover, since the plasma is ejected from the cavity in a small amount of time, the certainty of ignition is low in some cases.